1. Field of the Invention
The present invention relates to a novel production method of 4,6-diaminoresorcin which is a monomer for polybenzobisoxazole (PBO). More particularly, it relates to a production method of 4,6-diaminoresorcin in which resorcin is-used as a starting material, any step of forming a halogen-containing compound is not required and the production of by-products is inhibited. Furthermore, the present invention relates also to a novel intermediate useful for the synthesis of 4,6-diaminoresorcin and a production method of this intermediate.
A PBO fiber is superior to conventional fibers in terms of various properties such as strength, modulus of elasticity, heat resistance and chemical resistance, as disclosed in Japanese Patent Publication No. 501452/1986, so that it is expected to apply this kind of fiber to various uses including structural materials and thermal insulating materials as a super fiber which is superior to aramid, and hence the fiber is considered to be an industrially extremely useful resin. 4,6-Diaminoresorcin is a monomer for the PBO, and therefore, it is important as a raw material for the PBO.
2. Description of the Related Art
A PBO is a polymer represented by the following general structural formula (c) and can be obtained through a condensation reaction between a compound (a) and a compound (b) as represented by the following reaction formula: wherein Ar is an aromatic group; and Y is a functional group having an electron-deficient carbon such as a carboxyl group, a carboxylic acid halide group, a haloalkyl group or a nitrile group.
The thus obtained polybenzobisoxazole is used as fibers, films and the like, but its physical properties such as strength and modulus of elasticity are greatly influenced by a polymerization degree of the polymer. It is known that in a polycondensation reaction, a maximum polymer viscosity is generally obtained when a feed ratio of the monomers is 1:1, and the polymer viscosity rapidly decreases as the feed ratio of the monomers deviates from a ratio of 1:1. That is, to attain a satisfactory sufficient polymerization degree, it is necessary to strictly control the feed ratio of the monomers.
However, in the case that the monomers contain impurities in large quantities, it is difficult to control this feed ratio of the monomers. Particularly, when the monomers contain even small amounts of monoamine and triamines which act as polymerization terminators, the deterioration of the polymerization degree is caused. Therefore, there have been desired the high-purity monomers containing neither the monoamine nor the triamines.
As a monomer (a) for the PBO, 4,6-diaminoresorcin is known, and several synthesis methods for this compound have been reported.
A conventional production method of 4,6-diaminoresorcin comprises synthesizing dinitroresorcin as a precursor by a method in which resorcin is acetylated and then nitrated (Ber. Dtsch. Chem. Ges., 16, 552, 1883), a method in which 1,3-bis(alkyl carbonate)benzene is nitrated (Japanese Patent Application Laid-Open No. 136/1990) or another method, and then reducing the thus synthesized dinitroresorcin.
However, the operation of this conventional method is complicated and a manufacturing cost increases, because protecting groups are introduced for the hydroxy groups of resorcin, and from an industrial viewpoint, the above method has a problem that the protecting groups eliminated in a hydrolysis step become an unrecoverable by-product, and a problem that a trinitro compound is produced in the nitration step and a triamino compound is produced in the reduction step, and they disturb the polymerization in the synthesis of the PBO.
Furthermore, several methods using no protecting groups have been proposed in which a halobenzene is used as a raw material, and there are known, for example, a method in which a trichlorobenzene is nitrated (Japanese Patent Application Laid-Open No. 500743/1990) and a method in which a dihalobenzene is nitrated and then hydrolyzed with an alkali (Japanese Patent Application Laid-Open Nos. 238561/1989, 233127/1995, 316102/1995 and 73417/1996).
In these methods, however, since 4,6-dinitroresorcin is unstable under the alkali conditions in the hydrolysis step, the operation of these methods is apt to be complicated in order to avoid the decomposition of produced 4,6-dinitroresorcin. Furthermore, trichlorobenzene and its nitrated compound have a problem that they are strongly poisonous and cause an irritation on skin. Therefore, it is not preferable in consideration of the safety of an operator to pass through the production of a halogen-containing compound such as the halobenzene and its nitrated compound.
In addition, another method which comprises subjecting an aniline to diazotization and diazo-coupling the thus diazotized compound to resorcin, followed by hydrocracking is disclosed in Japanese Patent Application Laid-Open Nos. 242604/1995 and 124575/1997. In this method, however, aniline produced by the hydrocracking might be mixed with the product, and might disturb the polymerization in the synthesis of the PBO.